This invention relates to a core wire for a filament adapted to be heated for driving to emit electrons therefrom, such a filament and a method for mounting such a filament. Also, the present invention relates to a fluorescent display device for carrying out desired display due to impingement of electrons on phosphors and a method for manufacturing such a fluorescent display device. The fluorescent display devices include a write head for carrying out desired optical writing on a photosensitive element due to impingement of electrons emitted from a filament thereon.
Now, a conventional fluorescent display device of such a type as described above which has been generally known in the art will be described with reference to FIGS. 6 and 7, wherein FIG. 6 is a partially cutaway perspective view generally showing a structure of the fluorescent display device and FIG. 7 is a partially enlarged sectional view of the fluorescent display device of FIG. 6 which is taken in a direction in which filaments are arranged stretchedly or while being stretched.
The conventional fluorescent display device generally designated at reference numeral 31 in FIGS. 6 and 7, as shown in FIG. 6, includes a box-like envelope 32 of which an interior is kept at a high vacuum. The envelope 32 includes a box-like casing 36 which is constituted by an anode substrate 33 made of an insulating material, a front cover 34 made of an insulating and light-permeable material and a frame-like side plate 35 made of an insulating material.
The anode substrate 33, as shown in FIG. 7, is formed on an inner surface thereof positioned in the envelope 32 with a wiring layer (wiring conductor) 37 in a predetermined pattern depending on a display pattern 38. The wiring layer 37 may be made of, for example, an thin Al film. The wiring layer 37 is formed thereon with an insulating layer 39 laminatedly or by lamination. The insulating layer 39 is formed at portions thereof corresponding to display segments 40 of the display pattern 38 with through-holes 41, via which the wiring layer 37 is exposed. The through-holes 41 with which the wiring layer 37 is exposed each are filled therein with a conductive layer 42 by printing of a conductive paste.
The insulating layer 39, as shown in FIG. 7, is formed on a surface thereof facing the front cover 34 with anode conductors 43 in a discrete manner in correspondence to the display segments 40 of the display pattern 38 so as to be electrically connected to the wiring layer 37 through the conductive layers 42. The anode conductors 43 may be formed of a thick film by printing. The thick film may be made of, for example, either a graphite paste constituted by a graphite powder and an organic binder or an Al paste. This provides anodes 45 each for every display segments 40 of the display pattern 38.
Above the anodes 45, as shown in FIGS. 6 and 7, are arranged mesh-like grids 46, above which cathodes 47 each constituted by a filament are arranged stretchedly or while being stretched.
The filament for each of the cathodes 47 arranged in the fluorescent display device 31 is made by coating carbonate of Ba, Sr or Ca on a tungsten core wire of from several microns to tens of microns in diameter. In order to permit each of the filaments to emit thermions, it is required to feed the tungsten with electricity to heat the filament to a temperature within a range of between 600xc2x0 C. and 650xc2x0 C. For example, heating of the filament of, for example, 10 mm in length to a temperature of 650xc2x0 C. supposing that it is at a room temperature (24xc2x0 C.) during non-heating causes the filament to elongate by about 0.3 mm because tungsten has a thermal expansion coefficient of 4.4xc3x9710xe2x88x927. Such elongation of the filament by about 0.3 mm causes sagging of the filament, resulting in the filament contacting with the anodes 45 and grids 46 in the envelope 32, leading to a failure in insulation. Also, such sagging of the filament causes vibration of the filament when the fluorescent display device is of the vehicle-mounted type or the like, to thereby adversely affect display of the fluorescent display device. More particularly, vibration of the filament acting as an electron emitting source causes display of the fluorescent display device to be vibrated in synchronism therewith, leading to flickering of the display, to thereby deteriorate visibility of the display.
In order to eliminate the above-described problem, the conventional fluorescent display device 31 is provided with a support structure for fixing each of the filaments while applying tension of a predetermined magnitude to the filament, to thereby prevent sagging of the filament due to thermal expansion thereof. The support structure is constituted by a support 51 acting as a stationary stretching member for fixing one end of each of the filaments and an anchor 52 acting as an elastic stretching member for elastically holding the other end of each of the filaments. This permits each of the filaments to be fixed at one end thereof on the support 51 and elastically held at the other end thereof on the anchor 52, so that the filament may be elastically arranged between the support 51 and the anchor while being stretched therebetween.
Also, in place of the anchor 52 for elastically holding the filament, a structure which includes a coiled filament constituted by a coiled tungsten core wire formed at at least a part thereof with a coiled portion and a carbonate material electro-deposited on the coiled tungsten core wire, to thereby exhibit elasticity is known in the art. Such a coiled filament may be made as shown in FIG. 8. More specifically, the coiled tungsten core wires each are fixed on a metal frame 62 while keeping a coiled portion 61a thereof stretched and then the metal frame 62 is placed in an electrolytic bath 64 in which an electrolytic carbonate solution 63 is filled. Then, a voltage at a predetermined level such as, for example, 60V is applied between the metal frame 62 and the bath 64 for a predetermined period of time (for example, 60 seconds). This permits the carbonate to be electro-deposited on the coiled tungsten core wires, resulting in coiled filaments 61 being provided. Then, the coiled filaments 61 thus provided each are welded to a metal member for fixing the filament.
Unfortunately, the above-described approaches each have a disadvantage. More particularly, the elastic anchor 52 of FIG. 7 for absorbing sagging of the filament is expensive because it is required to subject a material for the anchor to etching and pressing. Also, manufacturing of the anchor 52 requires a holding plate for forming the anchor 52 into a frame-like shape. The holding plate must be formed into a thickness as large as 0.2 to 0.3 mm in order to overcome tension of the filament. Such an increased thickness causes the holding plate to be unsuitable for incorporation in an ultrathin fluorescent display device in which a distance between two substrates is as small as about 1 mm.
For example, use of a substrate material (for each of the anode substrate, front cover, side plate and the like) for the purpose of realizing an ultrathin fluorescent display device which has never been seen in the art causes a size of a package which permits the substrate to satisfactorily exhibit pressure resistance to be subjected to a restriction. Thus, it is required to arrange struts in the envelope. Also, the ultrathin fluorescent display device in which such thin substrates are arranged causes even a difference in level due to a thickness of the metal frame to apply stress to each of the substrates, leading to likelihood that a portion of the substrate through which the metal frame is led out cracks. Thus, it is required that the side plate is recessed at a portion thereof corresponding to a position of the flourescent display device of which the metal frame is led out by a depth corresponding to a thickness of the metal frame by chipping or the like.
The above-described coiled filaments 61 shown in FIG. 8 are made by fixing the plural coiled tungsten core wires in the metal frame 62 and then electro-depositing the carbonate on the core wires. This requires that the electro-deposition is carried out by batch processing for every metal frame 62 rather than continuous processing, leading to a failure in automation of manufacturing of the filaments. Also, welding of the filaments must be carried out for every filament after they are removed from the metal frame 62, to thereby further keep the manufacturing from being automated.
Further, in manufacturing of the coiled filaments 61, it is required to stretchedly arrange each of the coiled tungsten core wires while keeping the coiled portion 61a elongated in order to ensure application of tension at a predetermined level thereto.
Moreover, the welding is carried out with respect to the coiled tungsten core wire having the carbonate electro-deposited all over a surface thereof. Thus, the welding causes scattering of the carbonate in the envelope of the fluorescent display device, resulting in pollution therein, leading to a reduction in vacuum in the envelope or fluorescent display device. In addition, intrusion of the scattered carbonate into a display region of the fluorescent display device leads to a failure in display.
The present invention has been made in view of the foregoing disadvantage of the prior art.
Accordingly, it is an object of the present invention to provide a core wire for a filament or a filament core wire which is capable of being satisfactorily mounted while ensuring application of tension at a predetermined level thereto without using any expensive anchor and without applying unnecessary tension thereto.
It is another object of the present invention to provide a core wire for a filament or a filament core wire which is capable of attaining automation of production thereof.
It is a further object of the present invention to provide a filament core wire which is capable of being accommodated to an ultrathin fluorescent display device.
It is still another object of the present invention to provide a filament including such a filament core wire.
It is yet another object of the present invention to provide a method for mounting such a filament.
It is a still further object of the present invention to provide a fluorescent display device having such filaments incorporated therein.
In accordance with one aspect of the present invention, a core wire for a filament or a filament core wire is provided. The filament core wire includes a wire body provided at a part thereof with at least a spring section and a heat-decomposable block element arranged so as to cover the spring section while applying tension to the spring section.
In accordance with another aspect of the present invention, a filament is provided. The filament includes a filament core wire including a spring section provided thereon in proximity to at least one end thereof and at least a welded section arranged outside the spring section. The spring section is covered with a heat-decomposable block element arranged while applying tension to the spring section. The filament core wire has an electron emitting material deposited on a surface of a portion thereof which is not covered with the block element. The filament core wire is mounted on a mounting position by fixing it to a fixing spot of the mounting position.
In accordance with a further aspect of the present invention, a method for mounting a filament is provided. The method includes the step of mounting a filament on a mounting position. The filament is constituted by a filament core wire including a spring section provided thereon in proximity to at least one end thereof and at least a welded section arranged outside the spring section. The spring section is covered with a heat-decomposable block element arranged while applying tension to the spring section. The filament core wire has an electron emitting material deposited on a surface of a portion thereof which is not covered with the block element. The method further includes the step of fixing the welded section to a fixing spot of the mounting position.
In accordance with still another object of the present invention, a fluorescent display device for carrying out desired display by impinging electrons emitted from a filament stretchedly arranged in a vacuum envelope on a phosphor is provided. The filament includes a filament core wire including a spring section provided thereon in proximity to at least one end thereof and at least a welded section arranged outside the spring section. The spring section is covered with a heat-decomposable block element arranged while applying tension to the spring section. The filament core wire has an electron emitting material deposited on a surface of a portion thereof which is not covered with the block element. The filament core wire is mounted on a mounting position by fixing the welded section to a fixing spot of the mounting position. The block element of the spring section is removed therefrom by heating to keep tension in the spring section.
In accordance with a still further aspect of the present invention, a method for manufacturing a fluorescent display device for carrying out desired display by impinging electrons emitted from a filament stretchedly arranged in a vacuum envelope on a phosphor is provided. The method includes the steps of forming a filament core wire of the filament with a spring section so as to be positioned in proximity to at least one end thereof, covering at least the spring section with a heat-decomposable block element while applying tension to the spring section, depositing an electron emitting material on a surface of a portion of the filament core wire which is not covered with the block element, fixing the welded section to a fixing spot of a mounting position, and heating the envelope in which the filament fixed to the fixing spot of the mounting position is received to seal and evacuate the envelope and remove the block element of the spring section therefrom by heating.